Average S, as used normally in the CN equation, calculated from the curve number, which is based on land-use. This is in units of depth, often mm or inches
field_capacity
field capacity - the amount of water that a soil can hold after drainage. (fraction)
soil_water_content
Soil water content (fraction) on a given day.
porosity
Saturated water content, approximately equal to the porosity of a soil (fraction)